The present invention relates to an objective lens for an optical pick-up used for recording data to and/or reproducing data from an optical disc, such as a blu-ray disc, having a higher recording density than a DVD.
Recently, objective lenses for an optical pick-up used for recording data to and/or reproducing data from an optical disc having a higher recording density than a DVD have been proposed. An example of such an objective lens is disclosed in Japanese Patent Provisional Publication No. 2003-85806A (hereafter, referred to as JP2003-85806A).
The objective lens disclosed in JP2003-85806A has a first surface on a light source side and a second surface on an optical disc side. The first and second surfaces of the objective lens are a convex surface and a concave surface, respectively. That is, the objective lens is a single-element lens having a meniscus shape. To cancel the spherical aberration caused by change of the thickness of the optical disc with change of the degree of divergence of a light beam incident on the objective lens, the objective lens is configured to have an offence against a sine condition exhibiting a positive local maximum value in a range of 60 to 90 percent of an effective radius of the objective lens.
Although the theoretical performance of the above mentioned objective lens may be excellent, the objective lens has a drawback that the sensitivity to the decentering amount between lens surfaces or an error in lens thickness caused during manufacturing of the objective lens is high. Therefore, even a small level of manufacturing errors is not permitted for achieving the theoretical performance of the objective lens. Consequently, manufacturing conditions of the objective lens becomes very strict.
In addition, the first surface of the above mentioned objective lens has a steep gradient in the periphery thereof, which further increases the difficulty of manufacturing of the objective lens.